1. Field of the Invention
The present invention relates to an apparatus for measuring a mass flow, in particular bulk material or a fluid, and the invention especially relates to an impeller arrangement used in a measuring device in which the mass flow rate is measured by using the Coriolis force measuring principle.
2. Discussion of the Related Art
Such a prior measuring device is known from our copending European patent application No. 96850202.1 filed on Nov. 29, 1996, describing the main principles for such a mass flow meter, and to which application herewith is referred.
In such a measuring device the particles to be measured are subjected to tangential acceleration in a whirling impeller, and torque exerted on the vanes of the impeller in reaction to the Coriolis force of accelerated particles is measured as an indication of the mass flow rate.
The object our above European patent application was to avoid measurement errors, due to uneven distribution in the inlet tube of the material to be measured.
The subject of this invention is another source of measuring error, which mainly occurs in the lower end of the measuring capacity of a Coriolis measuring device, i.e. at small flows of the material. This error is due to the air (gas) circulation through the impeller having the character of a fan. When accelerating the air its mass and also its whirling generate a torque on the motor. If this torque were constant at a certain speed of rotation of the axis of the motor it should be a part of the zero measuring signal together with that from the axis bearing friction. However that is not the case. The streaming air along the vanes of the impeller is also influencing the acceleration of the material particles which simultaneously are launched out from the center of the impeller. Thereby a flow measuring error occurs and this is especially noticeable as an non-linearity of the flow versus the measuring signal characteristic in its lower end.
It is known that from the patent application DE 1 211 415 to even out the air pressure difference between the upper compartment above the impeller and its underlying compartment. This method stabilises the air stream but does not influence its magnitude and therefore it will not influence the acceleration of the material along the vanes of the impeller.
It is also known from the patent U.S. Pat. No. 4,468,972, which deals with a flow meter for liquids, that the impeller consists of a circular floor on which a cup with a mainly circular shape has an upper circular opening and two opposing elongated portions terminating in opposing spouts (FIG. 4 and 5). By this arrangement the vane character of the impeller will become reduced. However, this design will only function for a liquid, as material particles which fall down along the circular part of the cup between the elongated portions are blocked from moving in radial direction and an amount of material will permanently remain in these areas. Even if such permanent collections of material would not disturb the function of the impeller the mass of that material will change the moment of inertia of the rotating impeller and thereby cause a change of the zero point of the output signal. Such error of stochastic character can not be eliminated. Another drawback with the curved shape of the vanes mean that the material particles sliding along a nonradially directed vane will get a lower acceleration and thereby an more unpredictable Coriolis force.